This application is based on and incorporates herein by reference Japanese Patent Applications No. 2001-133877 filed May 1, 2001, and No. 2002-3238 filed Jan. 10, 2002.
The present invention relates to a vehicle alternator that uses permanent magnets in a rotor and is mounted in passenger cars, trucks and the like.
It is proposed to use permanent magnets in a rotor of a vehicle alternating current generator (alternator) so that the alternator has less loss, because the permanent magnets have no excitation loss. However, as the permanent magnets generate fixed magnetic field, the output power generation of the alternator must be regulated by shorting output voltages generated by armature coils of the alternator (shorting control) or by cutting off a part of the output voltages (phase control). For these controls, electronic switching devices such as a silicon controlled rectifier (SCR) and a field effect transistor (FET) are used.
The SCR has a forward loss that is twice as large as that of a silicon diode, and does not withstand high temperatures. Therefore, SCRs are not suited to the alternators.
Field effect transistors (FETs), on the other hand, have less loss and withstand high temperatures as opposed to SCRs. However, as the FET has a parasitic diode, which allows electric current to flow, the output current of the alternator cannot be interrupted. Hence the electric power generation of the armature coils of the alternator cannot be regulated. Further, the FET does not withstand high voltages, and the output voltage generated by the armature coil often exceeds this withstanding voltage. Therefore, the FETs are likely to be damaged.
It is therefore an object of the present invention to provide a vehicle alternator that is capable of regulating its output power generation while using FETs.
According to the present invention, a vehicle alternator comprises a permanent magnet rotor, a stator having armature coils, and a rectifier circuit. In the rectifier circuit, FETs and low forward voltage diodes such as Schottky barrier diodes are connected in series so that the forward directions of the low forward voltage diodes and parasitic diodes of the MOSFETs are opposite. The low forward voltage diodes are connected between the MOSFETs and the armature coils.